Stereo-based calibration apparatus

ABSTRACT

A virtual reality (VR) headset calibration system calibrates a VR headset, which includes a plurality of locators and an inertial measurement unit (IMU) generating output signals indicative of motion of the VR headset. The system comprises a calibration controller configured to receive a headset model of the VR headset that identifies expected positions of each of the locators. The controller controls cameras to capture images of the VR headset while the headset is moved along a predetermined path. The images detect actual positions of the locators during the movement along the predetermined path. Calibration parameters for the locators are generated based on differences between the actual positions and the expected positions. Calibration parameters for the IMU are generated based on the calibration parameters for the locators and differences between expected and actual signals output by the IMU. The calibration parameters are stored to the VR headset.

BACKGROUND

The present disclosure generally relates to calibration systems, andmore specifically relates to calibrating virtual reality systems.

Virtual reality (VR) devices deliver media to users based at least inpart on the position and movement of a headset worn by a user. Thesedevices include components used to determine the position and movementof the headset. However, due to manufacturing tolerances and othervariations introduced during manufacturing or use of the VR headsets,these components may differ from an ideal model of the headset. Thus,cameras and other equipment that assess the location of these componentsmay not produce an output identical to the output expected from theheadset model. As a result of the differences between the actual andexpected outputs, the virtual reality device may not be able toaccurately track the position or motion of the headset. For example, thedifferences may cause the device to identify motion of the headset whenthere is none or to calculate incorrect rates of the motion. Theseinaccurate measurements may cause a dissonance between a user's actualmotion and media presented to the user via the headset.

SUMMARY

A headset calibration system calibrates components of a virtual reality(VR) headset to compensate for differences between headsets (e.g., dueto manufacturing tolerances), enabling a VR system associated with theVR headset to maintain tracking of the VR headset. In one embodiment,the components calibrated by the calibration system include locators onthe VR headset and an inertial measurement unit (IMU). A locator is anobject located in a specific position on the VR headset relative to oneor more other components, such as another locator, of the VR headset andrelative to a reference point on the VR headset. The IMU is a devicethat generates output signals indicative of motion of the VR headset,and may include various sensors to generate such signals, such asaccelerometers, gyroscopes, and other inertial measurement systems.

In one embodiment, the headset calibration system includes a pluralityof cameras, an imaging space imaged by the cameras, and a support armconfigured to couple to a VR headset and move the VR headset inpredetermined manners. For example, the support arm moves the VR headsetalong a predetermined path through the imaging space to calibrate thelocators, and moves the VR headset in one or more other predeterminedmanners to calibrate the IMU. The system also includes a calibrationcontroller configured to calibrate the VR headset by determiningdifferences between expected and actual positions of the locators on theVR headset and differences between expected and actual outputs of theIMU.

The calibration controller accesses a memory that stores a set ofexpected positions of the locators on the headset. The calibrationcontroller controls the cameras to capture images of the VR headsetwhile the headset is moved along the predetermined path. The capturedimages detect actual positions of the locators on the VR headset, whichmay differ from the expected positions due to manufacturing tolerances.Using images captured at each position of the VR headset, thecalibration controller identifies the respective position of the VRheadset in the imaging space and determines positions of at least asubset of the locators in the imaging space. The calibration controlleruses the determined position of each locator in the subset in theimaging space and the identified position of the VR headset to compute adifference between an actual position of each locator in the subset onthe VR headset and an expected position of the locator. Based on thecomputed differences, the calibration controller generates calibrationparameters for the locators. In one embodiment, the parameters for thelocators represent transformations that when applied to positions of thelocators in the imaging space to yield the expected positions of thelocators on the VR headset.

The calibration controller also generates calibration parameters for theIMU based on the calibration parameters for the locators and differencesbetween expected signals output by the IMU and actual signals output bythe IMU. In one embodiment, the headset calibration system performs botha static calibration (e.g., measuring signals output by the IMU whilethe headset is held at a specified position) and a dynamic calibration(e.g., measuring signals output by the IMU while the headset is moved).The calibration parameters for the IMU may represent transformationsthat when applied to actual signals output by the IMU yield the expectedsignals.

The calibration controller stores the calibration parameters for thelocators and the IMU to the headset. The calibration parameters can thenbe used during operation of the headset to accurately identify positionsand motion of the headset.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system environment in which a virtualreality console operates, in accordance with an embodiment.

FIG. 2A is a wire diagram of a virtual reality headset, in accordancewith an embodiment.

FIG. 2B is a wire diagram of a virtual reality headset including a frontrigid body and a rear rigid body, in accordance with an embodiment.

FIG. 3A is a schematic diagram of a headset calibration system, inaccordance with an embodiment.

FIG. 3B illustrates an example headset support, in accordance with anembodiment.

FIG. 4 is a block diagram of a calibration controller, in accordancewith an embodiment.

FIG. 5 illustrates an example triangulation of a position of a locator,in accordance with an embodiment.

FIGS. 6A-6B illustrate embodiments for specifying distances betweenactual and expected positions of locators, in accordance with anembodiment.

FIG. 7 is a flowchart illustrating a process for calibrating a VRheadset, in accordance with an embodiment.

FIGS. 8A-8C illustrate example orientations of a VR headset forcalibrating an inertial measurement unit (IMU), in accordance with anembodiment.

FIG. 9 illustrates an example rotation of a VR headset for calibratingan inertial measurement unit, in accordance with an embodiment.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles, or benefits touted, of the disclosure described herein.

DETAILED DESCRIPTION

Virtual Reality System Architecture

FIG. 1 is a block diagram of one embodiment of a virtual reality (VR)system environment 100 in which a VR headset 105 operates. The systemenvironment 100 shown by FIG. 1 comprises the VR headset 105, an imagingdevice 135, and a VR input interface 140 that are each coupled to a VRconsole 110. While FIG. 1 shows an example system 100 including one VRheadset 105, one imaging device 135, and one VR input interface 140, inother embodiments any number of these components may be included in thesystem 100. For example, there may be multiple VR headsets 105 eachhaving an associated VR input interface 140 and being monitored by oneor more imaging devices 135, with each VR headset 105, VR inputinterface 140, and imaging devices 135 communicating with the VR console110. In alternative configurations, different and/or additionalcomponents may be included in the system environment 100.

As further described below, the virtual reality console 110 receivesmovement information from the VR headset describing movement of the VRheadset 105 as determined by sensors on the VR headset 105. In addition,the virtual reality console 110 receives image information from theimaging device 135 showing the VR headset 105 as captured by the imagingdevice 135. The position of the virtual reality headset 105 in space isdetermined using the movement information and the image information.Before using the movement information, the movement information may betranslated from observed movement information using calibration data forthe sensors that captured the data, in order to correct formanufacturing and other differences between different VR headsets. Inaddition, the image information is used to detect the position oflocators on the VR headset 105. Each of the locators may also differfrom headset to headset, and a set of calibration parameters are used toconvert the identified positions of the locators from the imaging deviceto account for the particular difference of the locators on the VRheadset 105 observed by the imaging device 135 relative to a model ofthe VR headset 105. Using these calibration parameters, the virtualreality console 110 may correct for differences between different VRheadsets and more accurately model the movement of the VR headset in avirtual reality system.

The VR headset 105 is a head-mounted display that presents media to auser. Examples of media presented by the VR head set include one or moreimages, video, audio, or some combination thereof. In some embodiments,audio is presented via an external device (e.g., speakers and/orheadphones) that receives audio information from the VR headset 105, theVR console 110, or both, and presents audio data based on the audioinformation. Example embodiments of the VR headset 105 are furtherdescribed below in conjunction with FIGS. 2A and 2B.

In various embodiments, the VR headset 105 may comprise one or morerigid bodies, which may be rigidly or non-rigidly coupled to each other.A rigid coupling between rigid bodies causes the coupled rigid bodies toact as a single rigid entity. In contrast, a non-rigid coupling betweenrigid bodies allows the rigid bodies to move relative to each other. Anembodiment of the VR headset 105 that includes two rigid bodies that arenon-rigidly coupled together is further described below in conjunctionwith FIG. 2B.

The VR headset 105 includes an electronic display 115, one or morelocators 120, one or more sensors 125, and an inertial measurement unit(IMU) 130. The electronic display 115 displays images to the user inaccordance with data received from the VR console 110. In variousembodiments, the electronic display 115 may comprise a single electronicdisplay or multiple electronic displays (e.g., a display for each eye ofa user). In some embodiments, the media provided to the electronicdisplay 115 for presentation to the user is pre-distorted to aid incorrection of one or more types of optical errors. Additionally, theoptical components may increase a field of view of the displayed mediathrough magnification or through another suitable method. For example,the field of view of the displayed media is such that the displayedmedia is presented using almost all (e.g., 110 degrees diagonal), and insome cases all, of the user's field of view.

The locators 120 are objects located in specific positions on the VRheadset 105 relative to one another and relative to a specific referencepoint on the VR headset 105. Each locator 120 provides a point on the VRheadset 105 that may be identified by an external camera, such as theimaging device 135, to determine the orientation and motion of the VRheadset. A locator 120 may be a light emitting diode (LED), a cornercube reflector, a reflective marker, a type of light source thatcontrasts with an environment in which the VR headset 105 operates, orsome combination thereof. In embodiments where the locators 120 areactive (i.e., an LED or other type of light emitting device), thelocators 120 may emit light in the visible band (˜380 nm to 750 nm), inthe infrared (IR) band (˜750 nm to 1 mm), in the ultraviolet band (10 nmto 380 nm), some other portion of the electromagnetic spectrum, or somecombination thereof.

In some embodiments, the locators are located beneath an outer surfaceof the VR headset 105, which is transparent to the wavelengths of lightemitted or reflected by the locators 120 or is thin enough to notsubstantially attenuate the wavelengths of light emitted or reflected bythe locators 120. Additionally, in some embodiments, the outer surfaceor other portions of the VR headset 105 are opaque in the visible band.Thus, the locators 120 may emit light in the IR band under an outersurface that is transparent in the IR band but opaque in the visibleband.

A model of the VR headset 105 (such as a CAD model used to manufacturethe headset) specifies the positions of the locators 120 on the headset105. Since each headset 105 ideally has locators 120 distributed in thesame pattern, the positions of at least a subset of the locators 120 canbe used during operation of the VR headset 105 to determine a positionof the headset in space. However, the actual positions of the locatorson the VR headset 105 may deviate from the model positions due tomanufacturing tolerances. For this and other reasons, the actual locatorpositions may vary between different headsets 105.

The IMU 130 is an electronic device that generates motion data based onmeasurement signals received from one or more of the sensors 125. Themotion data includes, for example, translational or rotational velocityand translational or rotational acceleration of the VR headset 105. Togenerate the motion data, a sensor 125 generates one or more measurementsignals in response to motion of the VR headset 105, and may measure theabsolute position or inertial movement of the sensor. Examples of thesensors 125 include: one or more accelerometers, one or more gyroscopes,one or more magnetometers, or any other suitable type of sensor, or somecombination thereof. The sensors 125 may be located external to the IMU130, internal to the IMU 130, or some combination thereof.

Based on the one or more measurement signals from one or more sensors125, the IMU 130 generates motion data indicating an estimated positionof the VR headset 105 relative to an initial position of the VR headset105. For example, the sensors 125 include multiple accelerometers tomeasure translational motion (forward/back, up/down, left/right) andmultiple gyroscopes to measure rotational motion (e.g., pitch, yaw,roll). In some embodiments, the IMU 130 rapidly samples the measurementsignals and calculates the estimated position of the VR headset 105 fromthe sampled data. For example, the IMU 130 integrates the measurementsignals received from the accelerometers over time to estimate avelocity vector and integrates the velocity vector over time todetermine an estimated position of a reference point (e.g., intermediateestimated position) on the VR headset 105. Alternatively, the IMU 130provides the sampled measurement signals to the VR console 110, whichdetermines the motion data. The reference point is a point that may beused to describe the position of the VR headset 105. The reference pointmay generally be defined as a point in space; however, in practice, thereference point is defined as a point within the VR headset 105 (e.g., acenter of the IMU 130).

Like the locators 120, the IMU 130 may vary between VR headsets 105 dueto manufacturing tolerances. For example, different sensors 125 mayoutput slightly different signals under the same conditions or the IMU130 may be offset by small amounts from the headset model.

In one embodiment, the IMU 130 receives one or more calibrationparameters from the memory 132 that compensate for differences betweenheadsets 105. As further discussed below, the calibration parameters areuniquely generated for each VR headset 105 and map the headset 105 to anideal headset model, reducing error in determining positions and ratesof movement of the headset 105. Based on a received calibrationparameter (e.g., IMU parameters), the IMU 130 may adjust its operation(e.g., change sample rate, etc.). In some embodiments, as furtherdescribed below, certain calibration parameters cause the IMU 130 tooffset an estimated position of the VR headset 105 to correct positionalerrors resulting from deviations of the headset 105 from the headsetmodel.

The imaging device 135 generates position data in accordance withcalibration parameters received from the VR console 110. Positioncalibration data includes one or more images showing observed positionsof the locators 120 that are detectable by the imaging device 135. Theimaging device 135 may include one or more cameras, one or more videocameras, any other device capable of capturing images including one ormore of the locators 120, or some combination thereof. Additionally, theimaging device 135 may include one or more filters (e.g., used toincrease signal to noise ration). The imaging device 135 is configuredto detect light emitted or reflected from locators 120 in a field ofview of the imaging device 135. In embodiments where the locators 120include passive elements (e.g., a retroreflector), the imaging device135 may include a light source that illuminates some or all of thelocators 120, which reflect the light towards the light source in theimaging device 135. Position calibration data is communicated from theimaging device 135 to the VR console 110.

The VR input interface 140 is a device that allows a user to send actionrequests to the VR console 110. An action request is a request toperform a particular action. For example, an action request may be tostart or end an application or to perform a particular action within theapplication. The VR input interface 140 may include one or more inputdevices. Example input devices include: a keyboard, a mouse, a gamecontroller, or any other suitable device for receiving action requestsand communicating the received action requests to the VR console 110. Anaction request received by the VR input interface 140 is communicated tothe VR console 110, which performs an action corresponding to the actionrequest. In some embodiments, the VR input interface 140 may providehaptic feedback to the user in accordance with instructions receivedfrom the VR console 110. For example, haptic feedback is provided whenan action request is received, or the VR console 110 communicatesinstructions to the VR input interface 140 causing the VR inputinterface 140 to generate haptic feedback when the VR console 110performs an action.

The VR console 110 provides media to the VR headset 105 for presentationto the user in accordance with information received from one or more of:the imaging device 135, the VR headset 105, and the VR input interface140. In the example shown in FIG. 1, the VR console 110 includes anapplication store 145, a tracking module 150, and a virtual reality (VR)engine 155. Some embodiments of the VR console 110 have differentmodules than those described in conjunction with FIG. 1. Similarly, thefunctions further described below may be distributed among components ofthe VR console 110 in a different manner than is described here.

The application store 145 stores one or more applications for executionby the VR console 110. An application is a group of instructions that,when executed by a processor, generates media for presentation to theuser. Media generated by an application may be in response to inputsreceived from the user via movement of the HR headset 105 or the VRinterface device 140. Examples of applications include: gamingapplications, conferencing applications, video playback application, orother suitable applications.

The tracking module 150 tracks movements of the VR headset 105 usingposition data from the imaging device 135 and/or motion data receivedfrom the IMU 130. In one embodiment, the tracking module 150 determinespositions of a reference point of the VR headset 105 using observedlocators from the position data, a model of the VR headset 105, andcalibration parameters of the locators. For example, the tracking module150 observes positions of the locators in the position data. Theobserved locators are identified, and calibration parameters for theobserved locators are retrieved from the memory 132 of the headset 105.The tracking module 150 applies the calibration parameters to theobserved positions to transform the observed positions to expectedpositions of the corresponding locators on the VR headset. The expectedpositions are then applied to the model of the VR headset 105 todetermine the position of the reference point.

In one embodiment, the tracking module 150 also determines positions ofa reference point of the VR headset 105 using the motion data.Additionally, in some embodiments, the tracking module 150 may useportions of the motion data, the position data, or some combinationthereof, to predict a future location of the headset 105. The trackingmodule 150 provides the estimated or predicted future position of the VRheadset 105 to the VR engine 155.

The VR engine 155 executes applications within the system environmentand receives position information, acceleration information, velocityinformation, predicted future positions, or some combination thereof, ofthe VR headset 105 from the tracking module 150. Based on the receivedinformation, the VR engine 155 determines media to provide to the VRheadset 105 for presentation to the user. For example, if the receivedinformation indicates that the user has looked to the left, the VRengine 155 generates media for the VR headset 105 that mirrors theuser's movement in a virtual environment. Additionally, the VR engine155 performs an action within an application executing on the VR console110 in response to an action request received from the VR inputinterface 140 and provides feedback to the user that the action wasperformed. The provided feedback may be visual or audible feedback viathe VR headset 105 or haptic feedback via the VR input interface 140.

FIG. 2A is a wire diagram of one embodiment of a virtual realityheadset. The VR headset 200 is an embodiment of the VR headset 105 andincludes a front rigid body 205 and a band 210. The front rigid body 205includes the electronic display 115 (not shown), the IMU 130, the one ormore sensors 125, and the locators 120. In the embodiment shown by FIG.2A, the sensors 125 are located within the IMU 130, and neither thesensors 125 nor the IMU 130 are visible to the user.

The locators 120 are located in fixed positions on the front rigid body205 relative to one another and relative to a reference point 215. Thereference to locators 120 in FIG. 2A includes some of the locators 120present on the headset, though additional locators 120 are present inthis example. In the example of FIG. 2A, the reference point 215 islocated at the center of the IMU 130. Each of the locators 120 emitlight that is detectable by the imaging device 135. Locators 120, orportions of locators 120, are located on a front side 220A, a top side220B, a bottom side 220C, a right side 220D, and a left side 220E of thefront rigid body 205 in the example of FIG. 2A.

FIG. 2B is a wire diagram of an embodiment of a VR headset 225 includinga front rigid body 205 and a rear rigid body 230. The VR headset 225shown in FIG. 2B, is an embodiment of the VR headset 105 where the frontrigid body 205 and the rear rigid body 230 are coupled together via theband 210. The band 210 is non-rigid (e.g., elastic), so the front rigidbody 205 is not rigidly coupled to the rear rigid body 210. Thus, therear rigid body 230 may move in relation to the front rigid body 205,and, specifically, move in relation to the reference point 215. In someembodiments, the rear rigid body 230 allows the VR console 110 tomaintain tracking of the VR headset 105, even if the front rigid body205 is not visible to the imaging device 135. Locators 120 on the rearrigid body 230 are located in fixed positions relative to one anotherand relative to the reference point 215 on the front rigid body 205. Inthe example of FIG. 2B, one or more locators 120, or portions oflocators 120, on the rear rigid body 230 are located on a front side235A, a top side 235B, a bottom side 235C, a right side 235D, and a leftside 235E of the rear rigid body 230.

Virtual Reality Headset Calibration System

FIG. 3A is a schematic diagram illustrating a headset calibration system300, according to one embodiment. In one embodiment, as shown in FIG.3A, the headset calibration system 300 includes an imaging space 310 anda calibration controller 350 controlling operation of the headsetcalibration system 300. The imaging space 310 includes a plurality ofcameras 315 and a support arm 320 supporting a VR headset 105. Thesupport arm 320 and/or the cameras 315 move in predetermined mannerswhile the cameras 315 image the VR headset 105. Based on the imagescaptured by the cameras 315, the headset calibration system 300generates calibration parameters for the VR headset 105 that are usedduring operation of the VR headset 105 to locate the headset in spaceand determine the headset's movement. These calibration parametersinclude parameters related to the positions of the locators, whichcompensate for differences between expected (or model) positions andactual positions of the locators 120 of a headset 105. The calibrationparameters also include parameters related to static and dynamic outputsof the IMU 130. For the static outputs, the calibration parametersrepresent differences between expected and actual signals output fromthe IMU 130 while the headset 105 is static. For the dynamic outputs,the calibration parameters represent differences between expected andactual signals output from the IMU 130 while the headset 105 is rotatedor otherwise moved.

The support arm 320 is configured to attach to or otherwise support a VRheadset 105 within the imaging space 310. In one embodiment, the supportarm 320 includes clamps, screws, clips, or other fasteners to attach theVR headset 105 to the support arm 320. For example, a clamp connected tothe support arm 320 clamps to the top side 220B and bottom side 220C ofa VR headset 105, or clamps to a right side 220D and a left side 220E ofthe VR headset 105. In the example of FIG. 3A, the support arm 320 isattached to a back face of the headset 105, opposite the front face220A. Alternatively, a top of the support arm 320 may have a cavityshaped to receive a VR headset 105 to support the headset, without thesupport arm 320 fastening to the headset. Furthermore, for embodimentsof the headset 105 having both a front rigid body 205 and a rear rigidbody 230, the support arm 320 may support the front rigid body 205 whilea second support arm (not shown) supports the rear rigid body 230. Thesecond support arm fixes a position of the rear rigid body 230 withrespect to the front rigid body 205, enabling the headset calibrationsystem 300 to also calibrate the positions of the locators 120 on therear rigid body 230.

The support structures of the support arm 320 may be configured tosupport the VR headset 105 without occluding the locators 120. Forexample, the support arm 320 clamps to a portion of the headset 105 thatdoes not include locators 120, so that the clamps do not block imagingof the locators 120. The support arm 320 may also support a cableconfigured to electronically couple the VR headset 105 to thecalibration system 300. Via the cable, the headset calibration system300 receives information from the VR headset 105, such as signals outputfrom the IMU 130, and sends information to the VR headset 105, such asthe calibration parameters generated by the calibration system 300.

An example headset support 350, which is coupled to the support arm 320in the headset calibration system 300 to support the headset 105, isshown in FIG. 3B. In one embodiment, as illustrated in FIG. 3B, theheadset support 350 includes front support structures 352A and 352B tosupport the front rigid body 205 of the headset 105. For example, a topof the front rigid body 205 rests on the support structure 352A, while abottom of the front rigid body 205 rests on the support structure 352B.The band 210 of the headset 105 may be placed around a band support 354to secure the headset 105 to the support 350. A clamp (not shown in FIG.3B) may clamp to the inside of the front rigid body 205 to furthersecure the headset 105 to the support 350.

In one embodiment, the support arm 320 is coupled to actuatorsconfigured to move the support arm 320 and attached VR headset 105 inone or more predetermined manners within the headset calibration system300. Moving the headset 105 in a predetermined manner enables theheadset calibration system 300 to reliably compare actual positions ofthe locators 120 to expected locator positions and compare actual IMU130 outputs to expected outputs. Given accurate comparisons betweenthese values, the headset calibration system 300 can generatecalibration parameters that improve the accuracy of the tracking module150 when tracking a headset 105 in use. To calibrate positions of thelocators 120 of a VR headset 105, the actuators move the support arm 320along a fixed path 325. FIG. 3A illustrates one embodiment of the fixedpath 325 being a circular path, but the actuators may move the supportarm 320 along a path of any shape and oriented in any manner withrespect to the cameras 315. For example, the path may move the headset105 along a circular shape on a plane parallel to, perpendicular to, orat another angle to the imaging plane of the cameras 315, or along apath of any other shape. The path may move the headset 105 to providethe cameras a variety of perspectives and different views of the VRheadset 105 and the locators 120. Thus, the path may rotate, turn, andotherwise move the VR headset 105 to different positions along the path.The actuators move the support arm 320 at a specified rate along thefixed path. To calibrate the IMU 130, the actuators rotate the headset105 around one or more axes at a specified rate.

The cameras 315 capture images of the headset 105 while the headset isin the imaging space 310. The cameras 315 are configured such that thelocators 120 are visible in the captured images, and therefore thecameras 315 may image in the visible, infrared, or ultraviolet spectradepending on the type of locators 120 used in the headset 105. FIG. 3Aillustrates four cameras 315 positioned at corners and pointed toward acenter of the imaging space 310. However, in other embodiments, theheadset calibration system 300 may include additional or fewer cameras,and the cameras 315 may be located at different positions. For example,the cameras 315 and headset 105 may be approximately aligned on a commonhorizontal plane. Alternatively, the headset calibration system 300 mayinstead include cameras positioned above and below the headset 105 whenthe headset 105 is supported by the support arm 320. In one embodiment,the cameras 315 are positioned in the imaging space 310 to minimizeimaging error for the path 325. For example, the cameras 315 arepositioned such that any position along the path 325 is located withinan overlap of fields of view of at least two of the cameras.Furthermore, in one embodiment, the cameras 315 are coupled to actuatorsto move the cameras 315 in a predetermined manner relative to thesupport arm 320. Based on the images captured by the cameras, theheadset calibration system 300 determines actual positions of thelocators 120 relative to expected positions and determines actualoutputs from the IMU 130 relative to expected outputs.

The calibration controller 350, which is a computing device comprising aprocessor and a memory, controls the actuators of the support arm 320and the cameras 315 to move the headset 105 in the predetermined mannerand captured images of the headset. Using the images captured by thecameras 315, the calibration controller 350 generates calibrationparameters and stores the calibration parameters to the memory 132 ofthe headset 105.

FIG. 4 is a block diagram of the calibration controller 350, accordingto one embodiment. In one embodiment, the calibration controller 350includes an imaging module 405, a positioning module 410, a headsetcalibrator 415, and a communication module 420. Other embodiments of thecalibration controller 350 may include additional, fewer, or differentmodules, and the functions may be performed by different modules.

The imaging module 405 drives actuators to move the support arm 320and/or the cameras 315 in a predetermined manner. While the headset 105and/or cameras 315 are moved, the imaging module 405 controls thecameras 315 to capture images of the headset 105. In one embodiment, asdescribed below, the imaging module 405 controls the actuators and thecameras 315 differently for calibrating the positions of the locators120 and the outputs of the IMU 130.

The positioning module 410 uses the images captured by the imagingmodule 405 to triangulate positions of the locators 120 in the imagingspace 310. FIG. 5 illustrates an example of the triangulation processperformed by the positioning module 410. As shown in FIG. 5, two or moreof the cameras 315, such as cameras 315C and 315D, capture an image ofthe headset 105. Generally, the positioning module 410 uses knowngeometry of the cameras 315 with respect to the headset 105 when animage is captured to determine the positions of the locators 120 in theimaging space 310 and translate the positions in the imaging space toactual positions of the locators 120 on the headset 105. Morespecifically, the positioning module 410 identifies a position of alocator 120 in the images captured by two or more of the cameras 315.The positioning module 410 also determines the geometry of the cameras315, including distances of the cameras 315 from the headset 105 andangles between the cameras 315 and the headset 105, at the time theimages were captured. The camera geometry for each set of capturedimages may be predetermined and provided to the positioning module 405,or the positioning module 405 may determine the camera geometry based ontime stamps of the images and the predetermined movement of the cameras315 and/or the support arm 320. Using the camera geometry and theposition of a locator 120 in the captured images, the positioning module405 determines the actual position of the locator 120 on the headset105.

In one embodiment, the positioning module 410 also uses parameters ofthe cameras 315 when determining the positions of the locators 120.These parameters include, for example, focal length, focus, offset of animaging sensor with respect to the center of the lens of each camera315, or lens distortion parameters. For example, the cameras 315 eachhave a field of view 502, corresponding to a portion of the headset 105the camera captures in a given image. Lenses of the cameras 315 distortthe captured images, and the distortion is greater near edges of thefield of view 502 than near a center of the field of view. That is, thecamera distortion introduces an error into the measurement of the actualpositions of the locators 120, where the error is greater near the edgesof the field of vision 502 than near the center. The positioning module410 may use the camera distortions when determining the positions of thelocators 120 by adjusting the locators' observed positions to compensatefor the camera distortion.

As described above, the cameras 315 capture a plurality of images whilethe headset 105 and/or cameras 315 are moved in a predetermined manner.Accordingly, a given locator 120 may appear in multiple sets of imagescaptured by the cameras 315, where a set of images represents the imagessimultaneously captured by the plurality of cameras 315. In oneembodiment, the positioning module 410 triangulates the position of alocator 120 using any sets of images in which the locator 120 appears intwo or more of the images. In another embodiment, the positioning module410 selects one or more sets of images to use to determine the positionof a locator 120. For example, the positioning module 410 selects a setof images to use for positioning a locator 120 in which the locator 120is positioned closest to centers of the fields of vision of the cameras315. Alternatively, the positioning module 410 uses a set of images toposition any locators 120 falling within the fields of vision of thecameras 315. For example, in FIG. 5, the locators 120A and 120B arepositioned in the region of overlapping fields of view 502 of thecameras 315C and 315D, while the locators 120C and 120D are positionedoutside the overlapping region. The positioning module 410 thereforeuses the images captured in FIG. 5 to triangulate the positions of thelocators 120A and 120B, but does not use the images to triangulate thepositions of the locators 120C and 120D.

Returning to FIG. 4, the headset calibrator 415 generates calibrationparameters for the headset 105. Calibration parameters for a headset 105describe differences between expected positions and the actual positionsof the locators 120 and differences between expected outputs and actualoutputs of the IMU 130. For calibrating the positions of the locators120, the headset calibrator 415 determines distances between the actualpositions of the locators 120 (as determined by the positioning module410) and the expected positions of the locators 120 (as specified by amodel of the headset 105). In particular, the headset calibrator 415applies a non-linear least squares solver to the actual positions of thelocators 120 in selected images to determine the deviation of the actualposition of the locator 120 from the expected position.

FIGS. 6A-6B illustrate different embodiments for specifying the distancebetween actual and expected positions of locators 120. In FIG. 6A, theactual position 600 of a locator 120 is specified by a distance 602 fromthe reference point 215 and an angle 604 from a positive horizontal axis606 originating at the reference point 215. Similarly, the expectedposition 610 of the locator is specified by a distance 612 from thereference point 215 and an angle from the positive horizontal axis. Inthe embodiment of FIG. 6A, the calibration parameters are a vectortransformation that, when applied to the distance 612 and angleassociated with the expected position 610, outputs the distance 602 andangle 604 associated with the actual position 600. The actual andexpected positions of a locator 120 may alternatively be expressed usinga rectangular coordinate system or coordinate systems centered atpositions on the headset 105 other than the reference point 215. Inanother embodiment, as shown in FIG. 6B, the calibration parameter forthe locator 120 is a vector 622 specifying the actual position 600 ofthe locator as a distance and angle from the expected position 610. Theheadset calibrator 415 may alternatively express the actual position ofa locator relative to the expected position by any other suitablemethod.

To calibrate the signals output from the IMU 130, the headset calibrator415 calculates expected outputs from the IMU 130 and generatescalibration parameters that relate the expected outputs to the actualoutput of the IMU 130. The headset calibrator 415 determines theexpected outputs using images of the headset 105 and the calibrationparameters for the locators 120. Based on the images, the headsetcalibrator 415 calculates actual orientations and rates of movement ofthe headset 105, and determines the IMU signals that would correspond tothe calculated orientations and movements. These expected IMU 130signals are compared to the actual signals output by the IMU 130 togenerate the calibration parameters.

The communication module 420 communicates with the VR headset 105 toreceive data from the headset 105, such as output signals from the IMU130, and communicate data to the headset 105, such as the calibrationparameters generated for the headset 105. The communication module 420stores the generated calibration parameters to the memory 132 of theheadset for use during operation of the VR system 100.

Calibrating a Virtual Reality Headset

FIG. 7 is a flowchart illustrating a process for calibrating a VRheadset 105, according to one embodiment. In one embodiment, the stepsof the process are performed by the headset calibration system 300.Other embodiments may include additional, fewer, or different steps, andthe steps may be performed in different orders.

To calibrate a headset 105, the headset calibration system 300 captures702 a plurality of images of the headset 105 while the headset 105 ismoved along a predetermined path. The images are captured by a pluralityof cameras 315 imaging the predetermined path. In one embodiment, theheadset 105 is attached to a support arm 320, and actuators move thesupport arm 320 and attached headset 105 along the predetermined path.Moving the headset 105 along the path enables the headset calibrationsystem 300 to capture images of all locators 120 of the headset 105,regardless of the position of the locators on the headset. Accordingly,the path may be configured in any shape providing the cameras 315 withdirect paths for imaging various sides of the headset 105. In additionto or instead of moving the headset 105, the cameras 315 may be moved ina predetermined manner to image the headset 105. When capturing 702 theimages, the headset calibration system 300 may capture images of theheadset 105 at regular intervals while the headset is continuouslymoved, where the relative position of the headset 105 to the cameras 315in each captured image may be determined using a known rate of movementof the headset 105 and a duration of the intervals. Alternatively, theheadset calibration system 300 moves the headset 105 to a predeterminedposition on the path and pauses the motion while the cameras 315 eachcapture one or more images of the headset. The headset 105 is then movedto a next specified position and imaged.

Using the captured images, the headset calibration system 300 determines704 actual positions of locators 120 of the headset 105. The headsetcalibration system 300 determines positions of one or more locators ineach of the images of the headset 105. Based on the positions of thelocators in a set of at least two images captured at substantially thesame time (that is, while the headset 105 is located at a given positionon the path), the headset calibration system 300 determines thepositions of the locators 120 in the imaging space 310. In oneembodiment, the headset calibration system 300 selects the sets ofimages to use for positioning each locator 120 based the relativeposition of each locator 120 with respect to the fields of view of thecameras or based on any other relevant factors. Based on the position ofthe headset 105 in the imaging space 310 and the determined positions ofthe locators 120 in the space, the headset calibration system 300computes the actual positions of the locators on the headset 105.

The headset calibration system 300 generates 706 calibration parametersrelating the actual positions of the locators 120 to expected positionsof the locators 120. In one embodiment, the calibration parameters forthe locators are vector transformations that enable the imaging system135 to convert the detected positions of each locator 120 in an imagedspace to the expected positions of the locators 120 while the headset105 is in use. Thus, for each of a plurality of the locators 120, theheadset calibration system 300 compares the actual position of thelocator, detected by imaging and triangulation, to the expected positionof the locator, specified by a model of the headset 105. Based on thecomparison, the headset calibration system 300 generates a vectortransformation that when applied to the actual position of the locatoryields the expected position of the locator.

In one embodiment, the headset calibration system 300 performs twocalibration processes for the IMU 130: one that is based on staticorientations of the headset 105, and one that is based on dynamicmovement of the headset 105. For example, the static calibration is usedto calibrate signals output by an accelerometer of the IMU 130, whilethe dynamic calibration is used to calibrate signals output by agyroscope of the IMU 130. To perform a static calibration of the IMU130, the headset calibration system 300 positions the headset 105 in aplurality of orientations and captures 708 a plurality of images of theheadset 105 while the headset 105 is in each of the orientations. In oneembodiment, the headset 105 is moved to a variety of orientationsexpected to yield IMU 130 signals with a wide range of magnitudes.

FIGS. 8A-C illustrate three example orientations. In FIG. 8A, theheadset 105 is positioned horizontally with respect to gravity (e.g.,such that a magnitude of a z-axis accelerometer signal is expected to belarge while x- and y-axis accelerometer signals are expected to bezero). In FIG. 8B, the headset 105 has been rotated around the y-axis toan orientation expected to yield approximately equivalent, positivemagnitude x- and z-axis accelerometer signals and y-axis accelerometersignal of approximately zero magnitude. In FIG. 8C, the headset 105 hasbeen rotated further around the y-axis to an orientation expected toyield an x-axis signal with a high magnitude and y- and z-axisaccelerometer signals approximately equal to zero. The headset 105 maybe similarly moved to orientations yielding negative magnitude x-axisand z-axis accelerometer signals and positive and negative magnitudey-axis signals. While the headset 105 is imaged, the headset calibrationsystem 300 receives 710 and stores magnitudes of signals output by thesensors 125 of the IMU 130.

The headset calibration system 300 calculates 712 expected outputs fromthe IMU 130 for each of the orientations of the headset 105. To identifyeach orientation, the headset calibration system 300 determines thepositions of the locators in the images of the orientation captured bythe cameras 315. The headset calibration system 300 matches each locatorin the images to a locator in the headset model using the calibrationparameters, and then determines the orientation of the headset modelthat would result in the positions of the locators in the images. Oncethe orientation of the headset 105 is determined, the headsetcalibration system 300 determines the expected accelerometer outputs(e.g., an expected magnitude of the signal from each accelerometerchannel) for the orientation. Because the positions of the locators 120for the specific headset 105 are calibrated, the headset calibrationsystem 300 does not need to use the driving of the actuators todetermine the orientation of the headset 105 at a given moment.

The headset calibration system 300 also calculates 714 the orientationsof the headset 105 based on the actual output signals received from theIMU 130 for each orientation of the headset 105. For example, theheadset calibration system 300 determines the orientation of the headset105 based on the magnitudes of the signal from each accelerometerchannel. The headset calibration system 300 uses the magnitudes todetermine angles between the accelerometer channels, which in turncorrespond to angles between the headset 105 and the axes.

The headset calibration system 300 compares the actual signals outputfrom the IMU 130 to the expected signals by comparing the actualorientations of the headset 105 (calculated using the images of theheadset 105) to the expected orientations of the headset 105 (calculatedbased on the output signals in each orientation). Based on thecomparison, the headset calibration system 300 generates 716 calibrationparameters relating the actual signal outputs to the expected signaloutputs. The calibration parameters for the static IMU 130 signalsrepresent one- or greater-dimensional vector transforms that can beapplied to actual signals output by the IMU 130 to yield the expectedsignals. In one embodiment, the headset calibration system 300 generatesa calibration parameter for each accelerometer channel that represents ascaling factor that can be applied to signals from the channel todetermine the expected signals from the channel (i.e., a one-dimensionalvector transform). In another embodiment, the headset calibration system300 generates one or more calibration parameters to be applied tomulti-dimensional accelerometer signals to determine correspondingexpected signals (e.g., a multi-dimensional vector transform). Togenerate these vector transforms, the headset calibration system 300 mayaverage the differences between the expected and actual IMU signalsdetermined for each of the plurality of orientations (e.g., generated asingle calibration parameter for each accelerometer channel).Alternatively, the headset calibration system 300 may generate multiplecalibration parameters for the IMU 130, each corresponding to a range oforientations of the headset 105. For example, if the maximum expectedmagnitude of the x-axis accelerometer signal is X_max (e.g., themagnitude when the x-axis accelerometer is parallel to the direction ofgravity), the headset calibration system 300 generates a firstcalibration parameter to be used when the x-axis signal has magnitudesbetween zero and X_max/2, and a second calibration parameter to be usedfor magnitudes greater than X_max/2.

To perform a dynamic calibration of the IMU 130, the headset calibrationsystem 300 rotates the headset 105 around one or more axes and captures718 a plurality of images of the headset 105 while the headset isrotated. In one embodiment, the headset 105 is rotated at a fixed rateby the actuators coupled to the support arm. FIG. 9 illustrates anexample rotation of the headset 105 around the y-axis (e.g., pitch). Theheadset 105 may be similarly rotated around the x-axis (e.g., roll) andz-axis (e.g., yaw). While the headset 105 is imaged, the headsetcalibration system 300 receives 720 and stores signals output by the IMU130.

The headset calibration system 300 calculates 722 expected outputs fromthe IMU 130 for the rotations of the headset 105. To calculate theexpected outputs, the headset calibration system 300 determinespositions of the locators 120 in images captured by the cameras 315 andadjusts the determined positions based on the calibration parameters ofthe locators 120. Using the adjusted locator positions and the headsetmodel, the headset calibration system 300 determines orientations of theheadset 105 in space and a time at which the headset 105 is in eachorientation. The headset calibration system 300 then calculates the rateof rotation using the orientation and time data. Expected outputs fromthe IMU 130 for the calculated rotation rate are then determined. Asimilar process is used to determine the rate of each rotation of theheadset 105 (e.g., clockwise and counter-clockwise rotations around eachof the x-, y-, and z-axes). Similarly to the static calibration of theIMU 130, the rate of rotation of the headset 105 is determined based onthe imaging of the headset 105 and the calibration parameters for thelocators 120. As such, the headset calibration system 300 does not needto rely upon known driving of the actuators to determine the rate of theheadset's rotation.

The headset calibration system also calculates 724 the rate of yaw,pitch, or roll of the headset 105 based on the actual output signalsreceived from the IMU 130. For example, the headset calibration system300 determines the rate of rotation around a given axis based on themagnitude of a corresponding gyroscope signal while the headset 105 isrotated. Alternatively, the headset calibration system 300 determinesthe rate of rotation using accelerometer signals.

Based on the rate of rotation calculated using the images and the rateof rotation calculated using the actual IMU signals, the headsetcalibration system 300 determines a difference between the expected IMU130 signals and the actual signals. For example, the headset calibrationsystem 300 determines a difference between the calculated rotationrates. The headset calibration system 300 then generates 726 one or moredynamic calibration parameters based on the determined difference. Inone embodiment, the dynamic calibration parameters include a parametercorresponding to a rotation around each of the three primary axes of theheadset 105. Each dynamic calibration parameter represents atransformation that, when applied to a signal output by the IMU 130while the headset 105 is rotated around the corresponding axis, yieldsthe corresponding expected output signal. An accurate rate of rotationof the headset 105 can then be determined based on the expected outputsignal.

The headset calibration system 300 stores 728 the calibration parametersfor the locators and the static and dynamic IMU calibration parametersto the memory 132 of the headset 105. During operation of the headset105, the VR console 110 and/or the IMU 130 retrieve the calibrationparameters from the memory 132 and use the parameters to accuratelytrack movements of the headset 105. For example, the VR console 110 usesthe locator calibration parameters to determine positions of thelocators in space based on images captured by the imaging device 135. Asanother example, one embodiment of the IMU 130 uses the static anddynamic IMU calibration parameters to correct errors in estimates of theposition of the headset 105. The VR console 110 may alternatively usethe IMU calibration parameters to determine positions and rates ofmovement of the headset 105.

SUMMARY

The foregoing description of the embodiments of the disclosure has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the disclosure to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

The foregoing description of the embodiments of the disclosure has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the disclosure to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

Some portions of this description describe the embodiments of thedisclosure in terms of algorithms and symbolic representations ofoperations on information. These algorithmic descriptions andrepresentations are commonly used by those skilled in the dataprocessing arts to convey the substance of their work effectively toothers skilled in the art. These operations, while describedfunctionally, computationally, or logically, are understood to beimplemented by computer programs or equivalent electrical circuits,microcode, or the like. Furthermore, it has also proven convenient attimes, to refer to these arrangements of operations as modules, withoutloss of generality. The described operations and their associatedmodules may be embodied in software, firmware, hardware, or anycombinations thereof.

Any of the steps, operations, or processes described herein may beperformed or implemented with one or more hardware or software modules,alone or in combination with other devices. In one embodiment, asoftware module is implemented with a computer program productcomprising a computer-readable medium containing computer program code,which can be executed by a computer processor for performing any or allof the steps, operations, or processes described.

Embodiments of the disclosure may also relate to an apparatus forperforming the operations herein. This apparatus may be speciallyconstructed for the required purposes, and/or it may comprise ageneral-purpose computing device selectively activated or reconfiguredby a computer program stored in the computer. Such a computer programmay be stored in a non-transitory, tangible computer readable storagemedium, or any type of media suitable for storing electronicinstructions, which may be coupled to a computer system bus.Furthermore, any computing systems referred to in the specification mayinclude a single processor or may be architectures employing multipleprocessor designs for increased computing capability.

Embodiments of the disclosure may also relate to a product that isproduced by a computing process described herein. Such a product maycomprise information resulting from a computing process, where theinformation is stored on a non-transitory, tangible computer readablestorage medium and may include any embodiment of a computer programproduct or other data combination described herein.

Finally, the language used in the specification has been principallyselected for readability and instructional purposes, and it may not havebeen selected to delineate or circumscribe the inventive subject matter.It is therefore intended that the scope of the disclosure be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of thedisclosure, which is set forth in the following claims.

What is claimed is:
 1. A method comprising: accessing a memory thatstores a set of expected positions of a plurality of locators on avirtual reality (VR) headset; capturing a plurality of images of the VRheadset in a plurality of positions as the VR headset is moved along apredetermined path through a space imaged by a plurality of cameras, thecaptured images detecting actual positions of the plurality of locatorson the VR headset; using images captured at each position of the VRheadset: identifying the respective position of the VR headset in thespace; determining positions of at least a subset of the locators in thespace based on the images; computing a difference between an actualposition of each locator in the subset on the VR headset and theexpected position of the respective locator based on the determinedposition of the locator in the space and the identified position of theVR headset; generating calibration parameters for the plurality oflocators based on the computed differences; and storing the calibrationparameters for the plurality of locators to the VR headset, thecalibration parameters usable to identify positions and motion of the VRheadset during use of the VR headset.
 2. The method of claim 1, whereindetermining the positions of the subset of locators in the spacecomprises, for a locator of the subset: triangulating the position inthe space of the locator using at least one image capturedsimultaneously by two or more of the cameras; and determining the actualposition of the locator based on the triangulated position of thelocator in the space and the identified position of the VR headset. 3.The method of claim 1, wherein the calibration parameters for theplurality of locators comprise a calibration parameter representing atransformation that when applied to a position of a locator in the spaceyields the expected position of the respective locator on the VRheadset.
 4. The method of claim 1, wherein the VR headset furthercomprises an inertial measurement unit (IMU) generating output signalsindicative of motion of the VR headset, the method further comprising:generating calibration parameters for the IMU based on the calibrationparameters for the plurality of locators and differences betweenexpected signals output by the IMU and actual signals output by the IMU;and storing the calibration parameters for the IMU to the VR headset,the calibration parameters for the IMU usable to identify motion of theVR headset during use of the VR headset.
 5. The method of claim 4,further comprising: capturing a second plurality of images of the VRheadset while the VR headset is moved in one or more predeterminedmanners; and determining positions of the VR headset in the space usingthe second plurality of images.
 6. The method of claim 5, wherein theone or more predetermined manners of movement comprise a rotation of theVR headset to each of a plurality of orientations in the space, andwherein generating the calibration parameters for the IMU comprises:identifying the orientations of the VR headset based on the capturedimages and the calibration parameters for the plurality of locators;receiving the actual signals output by the IMU at each of the pluralityof orientations of the VR headset; determining the expected IMU signalsbased on the identified orientations; and identifying for each of theorientations, the difference between the actual signals output from theIMU at the orientation and the expected signals.
 7. The method of claim6, wherein identifying the orientations of the VR headset comprises, foran orientation of the VR headset: determining positions of a pluralityof the locators of the VR headset in the space using at least one of thecaptured images; translating the determined positions of the locators inthe space to the expected positions of the corresponding locators basedon the calibration parameters; and determining the orientation based onthe expected positions.
 8. The method of claim 6, wherein thecalibration parameters for the IMU comprise a calibration parameterrepresenting a transformation that when applied to an actual signaloutput by the IMU yields an expected signal.
 9. The method of claim 5,wherein the one or more predetermined manners of movement comprisesrotating the VR headset around one or more axes of the VR headset, andwherein generating the calibration parameters for the IMU comprises:receiving actual signals output by the IMU while the VR headset isrotated; calculating a rate of rotation of the IMU around each of theone or more axes based on the second plurality of captured images andthe calibration parameters for the plurality of locators; determiningthe expected IMU signals based on the calculated rates of rotation; andidentifying for the rotation around each of the one or more axes, thedifference between the actual signals and the expected signals.
 10. Asystem comprising: a plurality of cameras; an imaging space imaged bythe plurality of cameras; a support arm configured to couple to avirtual reality (VR) headset and move the VR headset along apredetermined path within the imaging space, the VR headset including aplurality of locators; and a calibration controller configured to:accessing a memory that stores a set of expected positions of each ofthe locators on the VR headset; control the cameras to capture aplurality of images of the VR headset in each of a plurality ofpositions while the VR headset is moved along the predetermined path,the captured images detecting actual positions of the plurality oflocators on the VR headset; using images captured at each position ofthe VR headset: identify the respective position of the VR headset inthe space; determine positions of at least a subset of the locators inthe space based on the images; compute a difference between an actualposition of each locator in the subset on the VR headset and theexpected position of the respective locator based on the determinedposition of the locator in the space and the identified position of theVR headset; generate calibration parameters for the plurality oflocators based on the computed differences; and store the calibrationparameters for the plurality of locators to the VR headset, thecalibration parameters usable to identify positions of the VR headsetduring use of the VR headset.
 11. The system of claim 10, wherein thecalibration controller is configured to determine the positions of thesubset of locators in the space by, for a locator of the subset:triangulating the position in the space of the locator using at leastone image captured simultaneously by two or more of the cameras; anddetermining the actual position of the locator based on the triangulatedposition of the locator in the space and the identified position of theVR headset.
 12. The system of claim 10, wherein the calibrationparameters for the plurality of locators comprise a calibrationparameter representing a transformation that when applied to a positionof a locator in the space yields the expected position of the respectivelocator on the VR headset.
 13. The system of claim 10, wherein the VRheadset further comprises an inertial measurement unit (IMU) generatingoutput signals indicative of motion of the VR headset, and wherein thecalibration controller is further configured to: generate calibrationparameters for the IMU based on the calibration parameters for theplurality of locators and differences between expected signals output bythe IMU and actual signals output by the IMU; and store the calibrationparameters for the IMU to the VR headset, the calibration parameters forthe IMU usable to identify motion of the VR headset during use of the VRheadset.
 14. The system of claim 13, wherein the calibration controlleris further configured to: control the cameras to capture a secondplurality of images of the VR headset while the VR headset is moved inone or more predetermined manners; and determine positions of the VRheadset in the space using the second plurality of images.
 15. Thesystem of claim 13, wherein the one or more predetermined manners ofmovement comprise a rotation of the VR headset to each of a plurality oforientations in the space, and wherein the calibration controller isconfigured to generate the calibration parameters for the IMU by:identifying the orientations of the VR headset based on the capturedimages and the calibration parameters for the plurality of locators;receiving the actual signals output by the IMU at each of the pluralityof orientations of the VR headset; determining the expected IMU signalsbased on the identified orientations; and identifying for each of theorientations, the difference between the actual signals output from theIMU at the orientation and the expected signals.
 16. The system of claim15, wherein the calibration controller is configured to identify theorientations of the VR headset by: determining, for an orientation ofthe VR headset, positions of a plurality of the locators of the VRheadset in the space using at least one of the captured images;translating the determined positions of the locators in the space to theexpected positions of the corresponding locators based on thecalibration parameters; and determining the orientation based on theexpected positions.
 17. The system of claim 13, wherein the calibrationparameters for the IMU comprise a calibration parameter representing atransformation that when applied to an actual signal output by the IMUyields an expected signal.
 18. The system of claim 13, wherein the oneor more predetermined manners of movement comprises rotating the VRheadset around one or more axes of the VR headset, and whereingenerating the calibration parameters for the IMU comprises: receivingactual signals output by the IMU while the VR headset is rotated;calculating a rate of rotation of the IMU around each of the one or moreaxes based on the second plurality of captured images and thecalibration parameters for the plurality of locators; determining theexpected IMU signals based on the calculated rates of rotation; andidentifying for the rotation around each of the one or more axes, thedifference between the actual signals and the expected signals.
 19. Thesystem of claim 10, wherein the plurality of cameras each has a field ofview, and wherein the imaging space comprises regions of overlap of thefields of view of at least two of the cameras.
 20. The system of claim10, wherein the support arm couples to the VR headset without occludingthe locators.